Balloon power sources with a buoyancy trade-off

ABSTRACT

Example embodiments may facilitate altitude control by a balloon in a balloon network. An example method involves: (a) causing a balloon to operate in a first mode, wherein the balloon comprises an envelope, a high-pressure storage chamber, and a solar power system, (b) while the balloon is operating in the first mode: (i) operating the solar power system to generate power for the balloon and (ii) using at least some of the power generated by the solar power system to move gas from the envelope to the high-pressure storage chamber such that the buoyancy of the balloon decreases; (c) causing the balloon to operate in a second mode; and while the balloon is operating in the second mode, moving gas from the high-pressure storage chamber to the envelope such that the buoyancy of the balloon increases.

BACKGROUND

Unless otherwise indicated herein, the materials described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

Computing devices such as personal computers, laptop computers, tabletcomputers, cellular phones, and countless types of Internet-capabledevices are increasingly prevalent in numerous aspects of modern life.As such, the demand for data connectivity via the Internet, cellulardata networks, and other such networks, is growing. However, there aremany areas of the world where data connectivity is still unavailable, orif available, is unreliable and/or costly. Accordingly, additionalnetwork infrastructure is desirable.

SUMMARY

In one aspect, a computer-implemented method involves: (i) causing aballoon to operate in a first mode, wherein the balloon comprises anenvelope, a high-pressure storage chamber, and a solar power system;(ii) while the balloon is operating in the first mode: (a) operating thesolar power system to generate power for the balloon and (b) using atleast some of the power generated by the solar power system to move gasfrom the envelope to the high-pressure storage chamber such that thebuoyancy of the balloon decreases; (iii) causing the balloon to operatein a second mode; and (iv) while the balloon is operating in the secondmode, moving gas from the high-pressure storage chamber to the envelopesuch that the buoyancy of the balloon increases.

In another aspect, a non-transitory computer readable medium may havestored therein instructions that are executable by a computing device tocause the computing device to perform functions including: (i) causing aballoon to operate in a first mode, wherein the balloon comprises anenvelope, a high-pressure storage chamber, and a solar power system;(ii) while the balloon is operating in the first mode: (a) operating thesolar power system to generate power for the balloon; and (b) using atleast some of the power generated by the solar power system to move gasfrom the envelope to the high-pressure storage chamber such that thebuoyancy of the balloon decreases; (iii) causing the balloon to operatein a second mode; and (iv) while the balloon is operating in the secondmode, moving gas from the high-pressure storage chamber to the envelopesuch that the buoyancy of the balloon increases.

In a further aspect, a balloon includes: (i) a solar power systemconfigured to generate power for a balloon, wherein the ballooncomprises an envelope and a high-pressure storage chamber; and (ii) acontrol system that is configured to: (a) decrease buoyancy of theballoon via use of power generated by the solar power system to move gasfrom the envelope to the high-pressure storage chamber, wherein movementof gas from the envelope to the high-pressure storage chamber reduces avolume in which the moved gas is contained; and (b) increase thebuoyancy of the balloon via movement of gas from the high-pressurestorage chamber to the envelope.

In yet a further aspect, a computer implemented method may involve: (a)causing a balloon to operate in a first mode, wherein the ballooncomprises an envelope, a high-pressure storage chamber, and a solarpower system; (b) while the balloon is operating in the first mode: (i)operating the solar power system to generate power for the balloon; and(ii) using at least some of the power generated by the solar powersystem to move gas from the envelope to the high-pressure storagechamber; (c) causing the balloon to operate in a second mode; and (d)while the balloon is operating in the second mode, moving gas from thehigh-pressure storage chamber to the envelope

These as well as other aspects, advantages, and alternatives, willbecome apparent to those of ordinary skill in the art by reading thefollowing detailed description, with reference where appropriate to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram illustrating a balloon network,according to an example embodiment.

FIG. 2 is a block diagram illustrating a balloon-network control system,according to an example embodiment.

FIG. 3 is a simplified block diagram illustrating a high-altitudeballoon, according to an example embodiment.

FIG. 4 is a simplified block diagram illustrating a balloon network thatincludes super-nodes and sub-nodes, according to an example embodiment.

FIG. 5 is a flow chart illustrating a computer-implemented method,according to an example embodiment.

FIG. 6 is a combined operational state diagram illustrating thefunctions of a balloon that utilizes a fuel cell and a gas-flow systemfor altitude control, according to an example embodiment.

DETAILED DESCRIPTION

Example methods and systems are described herein. Any example embodimentor feature described herein is not necessarily to be construed aspreferred or advantageous over other embodiments or features. Theexample embodiments described herein are not meant to be limiting. Itwill be readily understood that certain aspects of the disclosed systemsand methods can be arranged and combined in a wide variety of differentconfigurations, all of which are contemplated herein.

Furthermore, the particular arrangements shown in the Figures should notbe viewed as limiting. It should be understood that other embodimentsmay include more or less of each element shown in a given Figure.Further, some of the illustrated elements may be combined or omitted.Yet further, an example embodiment may include elements that are notillustrated in the Figures.

1. Overview

Example embodiments may be implemented in the context of a data networkthat includes a plurality of balloons; for example, a mesh networkformed by high-altitude balloons deployed in the stratosphere. Sincewinds in the stratosphere may affect the locations of the balloons in adifferential manner, each balloon in an example network may beconfigured to change its horizontal position by adjusting its verticalposition (i.e., altitude). For instance, by adjusting its altitude, aballoon may be able find winds that will carry it horizontally (e.g.,latitudinally and/or longitudinally) to a desired horizontal location.

To function as a node in a balloon network, a balloon may consume asignificant amount of power. However, increasing the amount of powersupplied by a battery (e.g., a Lithium-ion battery) and/or theincreasing the number of batteries on a balloon will typically increasethe size and weight of the battery or batteries, which may beundesirable for various reasons. Accordingly, an exemplary embodimentmay include one or more altitude control mechanisms that also functionas supplemental power systems, and generate power as part of theprocesses to increase or decrease the altitude of the balloon.

In an example embodiment, a balloon may move gas (e.g., hydrogen oranother lighter-than-air gas) between its envelope and a high-pressurestorage chamber to decrease or increase its buoyancy. More specifically,the high-pressure storage chamber may be sized and configured such thatthe density of gas increases when the gas is moved from the envelope tothe high-pressure storage chamber. As such, the balloon may pump gas outof the envelope and into the high-pressure storage chamber to decreaseits buoyancy. Conversely, the balloon may move gas back into theenvelope from the high-pressure storage chamber to increase itsbuoyancy. This gas flow can be used as a source of power at night, andit increases buoyancy.

Altitude control via movement of gas between the high-pressure storagechamber and the envelope (referred to herein as a “gas-flow” altitudecontrol mechanism) may be configured so as to consume power during theday, when solar power is plentiful, and to generate power via altitudecontrol during the night. In particular, during the day, the balloon mayuse the gas-flow system for some or all altitudinal decreases, and useother altitude control techniques for some or all altitudinal increases.As such, gas may be stored in the high-pressure storage chamber duringthe day. Then, during the night, the balloon may use the gas-flow systemfor some or all altitudinal increases, and use another technique forsome or all altitudinal decreases. Thus, use of the gas-flow system mayconsume power during the day when solar power is more-readily available,and generate power during the night, when solar power is less usable.

In some embodiments, a balloon may also use a fuel cell for altitudecontrol. Fuel cells can produce power via the chemical reaction ofhydrogen and oxygen to produce water. Further, fuel cells can also berun in reverse to create hydrogen and oxygen from water. Accordingly, toincrease its altitude, a balloon may run its fuel cell in reverse so asto generate gas (e.g., hydrogen gas), which can then be moved into theenvelope to increase buoyancy. Further, to decrease its altitude, a fuelcell may be configured to use gas from the envelope to generate power(e.g., via the chemical reaction of hydrogen from the envelope andoxygen to produce water).

In a further aspect, a balloon may implement an altitude control processthat uses a fuel cell and a gas-flow system. Specifically, during theday, the balloon may use the gas-flow system to decrease its altitude,and operate a fuel cell in reverse to increase its altitude. Both thesealtitude-control techniques may consume power, but doing so during theday when more solar power is available. Further, the combination ofthese process creates and stores gas (e.g., hydrogen gas), which is thenavailable for consumption during the night. In particular, during thenight, the balloon may use the gas-flow system to increase its altitude,and operate the fuel cell to decrease its altitude. Thus, during thenight, the mechanisms for increasing and decreasing the balloon'saltitude may both generate power, which may be immediately used tooperate the balloon and/or may be used to recharge the balloon'sbattery.

2. Example Balloon Networks

In an example balloon network, the balloons may communicate with oneanother using free-space optical communications. For instance, theballoons may be configured for optical communications using ultra-brightLEDs (which are also referred to as “high-power” or “high-output” LEDs).In some instances, lasers could be used instead of or in addition toLEDs, although regulations for laser communications may restrict laserusage. In addition, the balloons may communicate with ground-basedstation(s) using radio-frequency (RF) communications.

In some embodiments, a high-altitude-balloon network may be homogenous.That is, the balloons in a high-altitude-balloon network could besubstantially similar to each other in one or more ways. Morespecifically, in a homogenous high-altitude-balloon network, eachballoon is configured to communicate with nearby balloons via free-spaceoptical links. Further, some or all of the balloons in such a network,may also be configured communicate with ground-based station(s) using RFcommunications. (Note that in some embodiments, the balloons may behomogenous in so far as each balloon is configured for free-spaceoptical communication with other balloons, but heterogeneous with regardto RF communications with ground-based stations.)

In other embodiments, a high-altitude-balloon network may beheterogeneous, and thus may include two or more different types ofballoons. For example, some balloons may be configured as super-nodes,while other balloons may be configured as sub-nodes. Some balloons maybe configured to function as both a super-node and a sub-node. Suchballoons may function as either a super-node or a sub-node at aparticular time, or, alternatively, act as both simultaneously dependingon the context. For instance, an example balloon could aggregate searchrequests of a first type to transmit to a ground-based station. Theexample balloon could also send search requests of a second type toanother balloon, which could act as a super-node in that context.

In such a configuration, the super-node balloons may be configured tocommunicate with nearby super-node balloons via free-space opticallinks. However, the sub-node balloons may not be configured forfree-space optical communication, and may instead be configured for someother type of communication, such as RF communications. In that case, asuper-node may be further configured to communicate with sub-nodes usingRF communications. Thus, the sub-nodes may relay communications betweenthe super-nodes and one or more ground-based stations using RFcommunications. In this way, the super-nodes may collectively functionas backhaul for the balloon network, while the sub-nodes function torelay communications from the super-nodes to ground-based stations.Other differences could be present between balloons in a heterogeneousballoon network.

FIG. 1 is a simplified block diagram illustrating a balloon network 100,according to an example embodiment. As shown, balloon network 100includes balloons 102A to 102F, which are configured to communicate withone another via free-space optical links 104. Balloons 102A to 102Fcould additionally or alternatively be configured to communicate withone another via RF links 114. Balloons 102A to 102F may collectivelyfunction as a mesh network for packet-data communications. Further,balloons 102A to 102F may be configured for RF communications withground-based stations 106 and 112 via RF links 108. In another exampleembodiment, balloons 102A to 102F could be configured to communicate viaoptical link 110 with ground-based station 112.

In an example embodiment, balloons 102A to 102F are high-altitudeballoons, which are deployed in the stratosphere. At moderate latitudes,the stratosphere includes altitudes between approximately 10 kilometers(km) and 50 km altitude above the surface. At the poles, thestratosphere starts at an altitude of approximately 8 km. In an exampleembodiment, high-altitude balloons may be generally configured tooperate in an altitude range within the stratosphere that has lowerwinds (e.g., between 5 and 20 miles per hour (mph)).

More specifically, in a high-altitude-balloon network, balloons 102A to102F may generally be configured to operate at altitudes between 17 kmand 25 km (although other altitudes are possible). This altitude rangemay be advantageous for several reasons. In particular, this layer ofthe stratosphere generally has mild wind and turbulence (e.g., windsbetween 5 and 20 miles per hour (mph)). Further, while the winds between17 km and 25 km may vary with latitude and by season, the variations canbe modelled in a reasonably accurate manner. Additionally, altitudesabove 17 km are typically above the maximum flight level designated forcommercial air traffic. Therefore, interference with commercial flightsis not a concern when balloons are deployed between 17 km and 25 km.

To transmit data to another balloon, a given balloon 102A to 102F may beconfigured to transmit an optical signal via an optical link 104. In anexample embodiment, a given balloon 102A to 102F may use one or morehigh-power light-emitting diodes (LEDs) to transmit an optical signal.Alternatively, some or all of balloons 102A to 102F may include lasersystems for free-space optical communications over optical links 104.Other types of free-space optical communication are possible. Further,in order to receive an optical signal from another balloon via anoptical link 104, a given balloon 102A to 102F may include one or moreoptical receivers. Additional details of example balloons are discussedin greater detail below, with reference to FIG. 3.

In a further aspect, balloons 102A to 102F may utilize one or more ofvarious different RF air-interface protocols for communicationground-based stations 106 and 112 via RF links 108. For instance, someor all of balloons 102A to 102F may be configured to communicate withground-based stations 106 and 112 using protocols described in IEEE802.11 (including any of the IEEE 802.11 revisions), various cellularprotocols such as GSM, CDMA, UMTS, EV-DO, WiMAX, and/or LTE, and/or oneor more propriety protocols developed for balloon-to-ground RFcommunication, among other possibilities.

In a further aspect, there may scenarios where RF links 108 do notprovide a desired link capacity for balloon-to-ground communications.For instance, increased capacity may be desirable to provide backhaullinks from a ground-based gateway, and in other scenarios as well.Accordingly, an example network may also include downlink balloons,which could provide a high-capacity air-ground link.

For example, in balloon network 100, balloon 102F could be configured asa downlink balloon. Like other balloons in an example network, adownlink balloon 102F may be operable for optical communication withother balloons via optical links 104. However, downlink balloon 102F mayalso be configured for free-space optical communication with aground-based station 112 via an optical link 110. Optical link 110 maytherefore serve as a high-capacity link (as compared to an RF link 108)between the balloon network 100 and a ground-based station 112.

Note that in some implementations, a downlink balloon 102F mayadditionally be operable for RF communication with ground-based stations106. In other cases, a downlink balloon 102F may only use an opticallink for balloon-to-ground communications. Further, while thearrangement shown in FIG. 1 includes just one downlink balloon 102F, anexample balloon network can also include multiple downlink balloons. Onthe other hand, a balloon network can also be implemented without anydownlink balloons.

In other implementations, a downlink balloon may be equipped with aspecialized, high-bandwidth RF communication system forballoon-to-ground communications, instead of, or in addition to, afree-space optical communication system. The high-bandwidth RFcommunication system may take the form of an ultra-wideband system,which provides an RF link with substantially the same capacity as theoptical links 104. Other forms are also possible.

Balloons could be configured to establish a communication link withspace-based satellites in addition to, or as an alternative to, aground-based communication link.

Ground-based stations, such as ground-based stations 106 and/or 112, maytake various forms. Generally, a ground-based station may includecomponents such as transceivers, transmitters, and/or receivers forcommunication via RF links and/or optical links with a balloon network.Further, a ground-based station may use various air-interface protocolsin order communicate with a balloon 102A to 102F over an RF link 108. Assuch, ground-based stations 106 and 112 may be configured as an accesspoint with which various devices can connect to balloon network 100.Ground-based stations 106 and 112 may have other configurations and/orserve other purposes without departing from the scope of the invention.

Further, some ground-based stations, such as ground-based stations 106and 112, may be configured as gateways between balloon network 100 andone or more other networks. Such ground-based stations 106 and 112 maythus serve as an interface between the balloon network and the Internet,a cellular service provider's network, and/or other types of networks.Variations on this configuration and other configurations ofground-based stations 106 and 112 are also possible.

2a) Mesh Network Functionality

As noted, balloons 102A to 102F may collectively function as a meshnetwork. More specifically, since balloons 102A to 102F may communicatewith one another using free-space optical links, the balloons maycollectively function as a free-space optical mesh network.

In a mesh-network configuration, each balloon 102A to 102F may functionas a node of the mesh network, which is operable to receive datadirected to it and to route data to other balloons. As such, data may berouted from a source balloon to a destination balloon by determining anappropriate sequence of optical links between the source balloon and thedestination balloon. These optical links may be collectively referred toas a “lightpath” for the connection between the source and destinationballoons. Further, each of the optical links may be referred to as a“hop” on the lightpath.

To operate as a mesh network, balloons 102A to 102F may employ variousrouting techniques and self-healing algorithms. In some embodiments, aballoon network 100 may employ adaptive or dynamic routing, where alightpath between a source and destination balloon is determined andset-up when the connection is needed, and released at a later time.Further, when adaptive routing is used, the lightpath may be determineddynamically depending upon the current state, past state, and/orpredicted state of the balloon network.

In addition, the network topology may change as the balloons 102A to102F move relative to one another and/or relative to the ground.Accordingly, an example balloon network 100 may apply a mesh protocol toupdate the state of the network as the topology of the network changes.For example, to address the mobility of the balloons 102A to 102F,balloon network 100 may employ and/or adapt various techniques that areemployed in mobile ad hoc networks (MANETs). Other examples are possibleas well.

In some implementations, a balloon network 100 may be configured as atransparent mesh network. More specifically, in a transparent balloonnetwork, the balloons may include components for physical switching thatis entirely optical, without any electrical involved in physical routingof optical signals. Thus, in a transparent configuration with opticalswitching, signals travel through a multi-hop lightpath that is entirelyoptical.

In other implementations, the balloon network 100 may implement afree-space optical mesh network that is opaque. In an opaqueconfiguration, some or all balloons 102A to 102F may implementoptical-electrical-optical (OEO) switching. For example, some or allballoons may include optical cross-connects (OXCs) for OEO conversion ofoptical signals. Other opaque configurations are also possible.Additionally, network configurations are possible that include routingpaths with both transparent and opaque sections.

In a further aspect, balloons in an example balloon network 100 mayimplement wavelength division multiplexing (WDM), which may help toincrease link capacity. When WDM is implemented with transparentswitching, physical lightpaths through the balloon network may besubject to the “wavelength continuity constraint.” More specifically,because the switching in a transparent network is entirely optical, itmay be necessary to assign the same wavelength for all optical links ona given lightpath.

An opaque configuration, on the other hand, may avoid the wavelengthcontinuity constraint. In particular, balloons in an opaque balloonnetwork may include the OEO switching systems operable for wavelengthconversion. As a result, balloons can convert the wavelength of anoptical signal at each hop along a lightpath. Alternatively, opticalwavelength conversion could take place at only selected hops along thelightpath.

Further, various routing algorithms may be employed in an opaqueconfiguration. For example, to determine a primary lightpath and/or oneor more diverse backup lightpaths for a given connection, exampleballoons may apply or consider shortest-path routing techniques such asDijkstra's algorithm and k-shortest path, and/or edge and node-diverseor disjoint routing such as Suurballe's algorithm, among others.Additionally or alternatively, techniques for maintaining a particularQuality of Service (QoS) may be employed when determining a lightpath.Other techniques are also possible.

2b) Station-Keeping Functionality

In an example embodiment, a balloon network 100 may implementstation-keeping functions to help provide a desired network topology.For example, station-keeping may involve each balloon 102A to 102Fmaintaining and/or moving into a certain position relative to one ormore other balloons in the network (and possibly in a certain positionrelative to the ground). As part of this process, each balloon 102A to102F may implement station-keeping functions to determine its desiredpositioning within the desired topology, and if necessary, to determinehow to move to the desired position.

The desired topology may vary depending upon the particularimplementation. In some cases, balloons may implement station-keeping toprovide a substantially uniform topology. In such cases, a given balloon102A to 102F may implement station-keeping functions to position itselfat substantially the same distance (or within a certain range ofdistances) from adjacent balloons in the balloon network 100.

In other cases, a balloon network 100 may have a non-uniform topology.For instance, example embodiments may involve topologies where balloonsarea distributed more or less densely in certain areas, for variousreasons. As an example, to help meet the higher bandwidth demands thatare typical in urban areas, balloons may be clustered more densely overurban areas. For similar reasons, the distribution of balloons may bedenser over land than over large bodies of water. Many other examples ofnon-uniform topologies are possible.

In a further aspect, the topology of an example balloon network may beadaptable. In particular, station-keeping functionality of exampleballoons may allow the balloons to adjust their respective positioningin accordance with a change in the desired topology of the network. Forexample, one or more balloons could move to new positions to increase ordecrease the density of balloons in a given area. Other examples arepossible.

In some embodiments, a balloon network 100 may employ an energy functionto determine if and/or how balloons should move to provide a desiredtopology. In particular, the state of a given balloon and the states ofsome or all nearby balloons may be input to an energy function. Theenergy function may apply the current states of the given balloon andthe nearby balloons to a desired network state (e.g., a statecorresponding to the desired topology). A vector indicating a desiredmovement of the given balloon may then be determined by determining thegradient of the energy function. The given balloon may then determineappropriate actions to take in order to effectuate the desired movement.For example, a balloon may determine an altitude adjustment oradjustments such that winds will move the balloon in the desired manner.

2c) Control of Balloons in a Balloon Network

In some embodiments, mesh networking and/or station-keeping functionsmay be centralized. For example, FIG. 2 is a block diagram illustratinga balloon-network control system, according to an example embodiment. Inparticular, FIG. 2 shows a distributed control system, which includes acentral control system 200 and a number of regional control-systems 202Ato 202B. Such a control system may be configured to coordinate certainfunctionality for balloon network 204, and as such, may be configured tocontrol and/or coordinate certain functions for balloons 206A to 206I.

In the illustrated embodiment, central control system 200 may beconfigured to communicate with balloons 206A to 206I via number ofregional control systems 202A to 202C. These regional control systems202A to 202C may be configured to receive communications and/oraggregate data from balloons in the respective geographic areas thatthey cover, and to relay the communications and/or data to centralcontrol system 200. Further, regional control systems 202A to 202C maybe configured to route communications from central control system 200 tothe balloons in their respective geographic areas. For instance, asshown in FIG. 2, regional control system 202A may relay communicationsand/or data between balloons 206A to 206C and central control system200, regional control system 202B may relay communications and/or databetween balloons 206D to 206F and central control system 200, andregional control system 202C may relay communications and/or databetween balloons 206G to 206I and central control system 200.

In order to facilitate communications between the central control system200 and balloons 206A to 206I, certain balloons may be configured asdownlink balloons, which are operable to communicate with regionalcontrol systems 202A to 202C. Accordingly, each regional control system202A to 202C may be configured to communicate with the downlink balloonor balloons in the respective geographic area it covers. For example, inthe illustrated embodiment, balloons 206A, 206F, and 206I are configuredas downlink balloons. As such, regional control systems 202A to 202C mayrespectively communicate with balloons 206A, 206F, and 206I via opticallinks 206, 208, and 210, respectively.

In the illustrated configuration, where only some of balloons 206A to206I are configured as downlink balloons, the balloons 206A, 206F, and206I that are configured as downlink balloons may function to relaycommunications from central control system 200 to other balloons in theballoon network, such as balloons 206B to 206E, 206G, and 206H. However,it should be understood that it in some implementations, it is possiblethat all balloons may function as downlink balloons. Further, while FIG.2 shows multiple balloons configured as downlink balloons, it is alsopossible for a balloon network to include only one downlink balloon.

Note that a regional control system 202A to 202C may in fact just beparticular type of ground-based station that is configured tocommunicate with downlink balloons (e.g. the ground-based station 112 ofFIG. 1). Thus, while not shown in FIG. 2, a control system may beimplemented in conjunction with other types of ground-based stations(e.g., access points, gateways, etc.).

In a centralized control arrangement, such as that shown in FIG. 2, thecentral control system 200 (and possibly regional control systems 202Ato 202C as well) may coordinate certain mesh-networking functions forballoon network 204. For example, balloons 206A to 206I may send thecentral control system 200 certain state information, which the centralcontrol system 200 may utilize to determine the state of balloon network204. The state information from a given balloon may include locationdata, optical-link information (e.g., the identity of other balloonswith which the balloon has established an optical link, the bandwidth ofthe link, wavelength usage and/or availability on a link, etc.), winddata collected by the balloon, and/or other types of information.Accordingly, the central control system 200 may aggregate stateinformation from some or all the balloons 206A to 206I in order todetermine an overall state of the network.

The overall state of the network may then be used to coordinate and/orfacilitate certain mesh-networking functions such as determininglightpaths for connections. For example, the central control system 200may determine a current topology based on the aggregate stateinformation from some or all the balloons 206A to 206I. The topology mayprovide a picture of the current optical links that are available inballoon network and/or the wavelength availability on the links. Thistopology may then be sent to some or all of the balloons so that arouting technique may be employed to select appropriate lightpaths (andpossibly backup lightpaths) for communications through the balloonnetwork 204.

In a further aspect, the central control system 200 (and possiblyregional control systems 202A to 202C as well) may also coordinatecertain station-keeping functions for balloon network 204. For example,the central control system 200 may input state information that isreceived from balloons 206A to 206I to an energy function, which mayeffectively compare the current topology of the network to a desiredtopology, and provide a vector indicating a direction of movement (ifany) for each balloon, such that the balloons can move towards thedesired topology. Further, the central control system 200 may usealtitudinal wind data to determine respective altitude adjustments thatmay be initiated to achieve the movement towards the desired topology.The central control system 200 may provide and/or support otherstation-keeping functions as well.

FIG. 2 shows a distributed arrangement that provides centralizedcontrol, with regional control systems 202A to 202C coordinatingcommunications between a central control system 200 and a balloonnetwork 204. Such an arrangement may be useful to provide centralizedcontrol for a balloon network that covers a large geographic area. Insome embodiments, a distributed arrangement may even support a globalballoon network that provides coverage everywhere on earth. Adistributed-control arrangement may be useful in other scenarios aswell.

Further, it should be understood that other control-system arrangementsare possible. For instance, some implementations may involve acentralized control system with additional layers (e.g., sub-regionsystems within the regional control systems, and so on). Alternatively,control functions may be provided by a single, centralized, controlsystem, which communicates directly with one or more downlink balloons.

In some embodiments, control and coordination of a balloon network maybe shared between a ground-based control system and a balloon network tovarying degrees, depending upon the implementation. In fact, in someembodiments, there may be no ground-based control systems. In such anembodiment, all network control and coordination functions may beimplemented by the balloon network itself. For example, certain balloonsmay be configured to provide the same or similar functions as centralcontrol system 200 and/or regional control systems 202A to 202C. Otherexamples are also possible.

Furthermore, control and/or coordination of a balloon network may bede-centralized. For example, each balloon may relay state informationto, and receive state information from, some or all nearby balloons.Further, each balloon may relay state information that it receives froma nearby balloon to some or all nearby balloons. When all balloons doso, each balloon may be able to individually determine the state of thenetwork. Alternatively, certain balloons may be designated to aggregatestate information for a given portion of the network. These balloons maythen coordinate with one another to determine the overall state of thenetwork.

Further, in some aspects, control of a balloon network may be partiallyor entirely localized, such that it is not dependent on the overallstate of the network. For example, individual balloons may implementstation-keeping functions that only consider nearby balloons. Inparticular, each balloon may implement an energy function that takesinto account its own state and the states of nearby balloons. The energyfunction may be used to maintain and/or move to a desired position withrespect to the nearby balloons, without necessarily considering thedesired topology of the network as a whole. However, when each balloonimplements such an energy function for station-keeping, the balloonnetwork as a whole may maintain and/or move towards the desiredtopology.

As an example, each balloon A may receive distance information d₁ tod_(k) with respect to each of its k closest neighbors. Each balloon Amay treat the distance to each of the k balloons as a virtual springwith vector representing a force direction from the first nearestneighbor balloon i toward balloon A and with force magnitudeproportional to d_(i). The balloon A may sum each of the k vectors andthe summed vector is the vector of desired movement for balloon A.Balloon A may attempt to achieve the desired movement by controlling itsaltitude.

Alternatively, this process could assign the force magnitude of each ofthese virtual forces equal to d_(i)×d_(I), wherein d_(I) is proportionalto the distance to the second nearest neighbor balloon, for instance.

In another embodiment, a similar process could be carried out for eachof the k balloons and each balloon could transmit its planned movementvector to its local neighbors. Further rounds of refinement to eachballoon's planned movement vector can be made based on the correspondingplanned movement vectors of its neighbors. It will be evident to thoseskilled in the art that other algorithms could be implemented in aballoon network in an effort to maintain a set of balloon spacingsand/or a specific network capacity level over a given geographiclocation.

2d) Example Balloon Configuration

Various types of balloon systems may be incorporated in an exampleballoon network. As noted above, an example embodiment may utilizehigh-altitude balloons, which could typically operate in an altituderange between 17 km and 25 km. FIG. 3 shows a high-altitude balloon 300,according to an example embodiment. As shown, the balloon 300 includesan envelope 302, a skirt 304, a payload 306, and a cut-down system 308,which is attached between the balloon 302 and payload 304.

The envelope 302 and skirt 304 may take various forms, which may becurrently well-known or yet to be developed. For instance, the envelope302 and/or skirt 304 may be made of a highly-flexible latex material ormay be made of a rubber material such as chloroprene. In one exampleembodiment, the envelope and/or skirt could be made of metalized Mylaror BoPet. Other materials are also possible. Further, the shape and sizeof the envelope 302 and skirt 304 may vary depending upon the particularimplementation. Additionally, the envelope 302 may be filled withvarious different types of gases, such as helium and/or hydrogen. Othertypes of gases are possible as well.

The payload 306 of balloon 300 may include a processor 312 and on-boarddata storage, such as memory 314. The memory 314 may take the form of orinclude a non-transitory computer-readable medium. The non-transitorycomputer-readable medium may have instructions stored thereon, which canbe accessed and executed by the processor 312 in order to carry out theballoon functions described herein.

The payload 306 of balloon 300 may also include various other types ofequipment and systems to provide a number of different functions. Forexample, payload 306 may include optical communication system 316, whichmay transmit optical signals via an ultra-bright LED system 320, andwhich may receive optical signals via an optical-communication receiver322 (e.g., a photodiode receiver system). Further, payload 306 mayinclude an RF communication system 318, which may transmit and/orreceive RF communications via an antenna system 340.

The payload 306 may also include a power supply 326 to supply power tothe various components of balloon 300. The power supply 326 couldinclude a rechargeable battery. In other embodiments, the power supply326 may additionally or alternatively represent other means known in theart for producing power. In addition, the balloon 300 may include asolar power generation system 327. The solar power generation system 327may include solar panels and could be used to generate power thatcharges and/or is distributed by power supply 326.

Further, payload 306 may include various types of other systems andsensors 328. For example, payload 306 may include one or more videoand/or still cameras, a GPS system, various motion sensors (e.g.,accelerometers, magnetometers, gyroscopes, and/or compasses), and/orvarious sensors for capturing environmental data. Further, some or allof the components within payload 306 may be implemented in a radiosondeor other probe, which may be operable to measure, e.g., pressure,altitude, geographical position (latitude and longitude), temperature,relative humidity, and/or wind speed and/or wind direction, among otherinformation.

As noted, balloon 300 includes an ultra-bright LED system 320 forfree-space optical communication with other balloons. As such, opticalcommunication system 316 may be configured to transmit a free-spaceoptical signal by modulating the ultra-bright LED system 320. Theoptical communication system 316 may be implemented with mechanicalsystems and/or with hardware, firmware, and/or software. Generally, themanner in which an optical communication system is implemented may vary,depending upon the particular application. The optical communicationsystem 316 and other associated components are described in furtherdetail below.

In a further aspect, balloon 300 may be configured for altitude control.For instance, balloon 300 may include a variable buoyancy system, whichis configured to change the altitude of the balloon 300 by adjusting thevolume and/or density of the gas in the balloon 300. A variable buoyancysystem may take various forms, and may generally be any system that canchange the volume and/or density of gas in the envelope 302.

In an example embodiment, a variable buoyancy system may include abladder 310 that is located inside of envelope 302. The bladder 310could be an elastic chamber configured to hold liquid and/or gas.Alternatively, the bladder 310 need not be inside the envelope 302. Forinstance, the bladder 310 could be a ridged bladder that could bepressurized well beyond neutral pressure. The buoyancy of the balloon300 may therefore be adjusted by changing the density and/or volume ofthe gas in bladder 310. To change the density in bladder 310, balloon300 may be configured with systems and/or mechanisms for heating and/orcooling the gas in bladder 310. Further, to change the volume, balloon300 may include pumps or other features for adding gas to and/orremoving gas from bladder 310. Additionally or alternatively, to changethe volume of bladder 310, balloon 300 may include release valves orother features that are controllable to allow gas to escape from bladder310. Multiple bladders 310 could be implemented within the scope of thisdisclosure. For instance, multiple bladders could be used to improveballoon stability.

In an example embodiment, the envelope 302 could be filled with helium,hydrogen or other lighter-than-air material. The envelope 302 could thushave an associated upward buoyancy force. In such an embodiment, air inthe bladder 310 could be considered a ballast tank that may have anassociated downward ballast force. In another example embodiment, theamount of air in the bladder 310 could be changed by pumping air (e.g.,with an air compressor) into and out of the bladder 310. By adjustingthe amount of air in the bladder 310, the ballast force may becontrolled. In some embodiments, the ballast force may be used, in part,to counteract the buoyancy force and/or to provide altitude stability.

In another embodiment, a portion of the envelope 302 could be a firstcolor (e.g., black) and/or a first material from the rest of envelope302, which may have a second color (e.g., white) and/or a secondmaterial. For instance, the first color and/or first material could beconfigured to absorb a relatively larger amount of solar energy than thesecond color and/or second material. Thus, rotating the balloon suchthat the first material is facing the sun may act to heat the envelope302 as well as the gas inside the envelope 302. In this way, thebuoyancy force of the envelope 302 may increase. By rotating the balloonsuch that the second material is facing the sun, the temperature of gasinside the envelope 302 may decrease. Accordingly, the buoyancy forcemay decrease. In this manner, the buoyancy force of the balloon could beadjusted by changing the temperature/volume of gas inside the envelope302 using solar energy. In such embodiments, it is possible that abladder 310 may not be a necessary element of balloon 300. Thus, variouscontemplated embodiments, altitude control of balloon 300 could beachieved, at least in part, by adjusting the rotation of the balloonwith respect to the sun.

Further, a balloon 306 may include a navigation system (not shown). Thenavigation system may implement station-keeping functions to maintainposition within and/or move to a position in accordance with a desiredtopology. In particular, the navigation system may use altitudinal winddata to determine altitudinal adjustments that result in the windcarrying the balloon in a desired direction and/or to a desiredlocation. The altitude-control system may then make adjustments to thedensity of the balloon chamber in order to effectuate the determinedaltitudinal adjustments and cause the balloon to move laterally to thedesired direction and/or to the desired location. Alternatively, thealtitudinal adjustments may be computed by a ground-based orsatellite-based control system and communicated to the high-altitudeballoon. In other embodiments, specific balloons in a heterogeneousballoon network may be configured to compute altitudinal adjustments forother balloons and transmit the adjustment commands to those otherballoons.

As shown, the balloon 300 also includes a cut-down system 308. Thecut-down system 308 may be activated to separate the payload 306 fromthe rest of balloon 300. The cut-down system 308 could include at leasta connector, such as a balloon cord, connecting the payload 306 to theenvelope 302 and a means for severing the connector (e.g., a shearingmechanism or an explosive bolt). In an example embodiment, the ballooncord, which may be nylon, is wrapped with a nichrome wire. A currentcould be passed through the nichrome wire to heat it and melt the cord,cutting the payload 306 away from the envelope 302.

The cut-down functionality may be utilized anytime the payload needs tobe accessed on the ground, such as when it is time to remove balloon 300from a balloon network, when maintenance is due on systems withinpayload 306, and/or when power supply 326 needs to be recharged orreplaced.

In an alternative arrangement, a balloon may not include a cut-downsystem. In such an arrangement, the navigation system may be operable tonavigate the balloon to a landing location, in the event the balloonneeds to be removed from the network and/or accessed on the ground.Further, it is possible that a balloon may be self-sustaining, such thatit does not need to be accessed on the ground. In other embodiments,in-flight balloons may be serviced by specific service balloons oranother type of aerostat or aircraft.

In a further aspect, balloon 300 includes a gas-flow system, which maybe used for altitude control. In the illustrated example, the gas-flowsystem includes a high-pressure storage chamber 342, a gas-flow tube350, and a pump 348, which may be used to pump gas out of the envelope302, through the gas-flow tube 350, and into the high-pressure storagechamber 342. As such, balloon 300 may be configured to decrease itsaltitude by pumping gas out of envelope 302 and into high-pressurestorage chamber 342. Further, balloon 300 may be configured to move gasinto the envelope and increase its altitude by opening a valve 352 atthe end of gas-flow tube 350, and allowing lighter-than-air gas fromhigh-pressure storage chamber 342 to flow into envelope 302.

Note that the high-pressure storage chamber in an example balloon, mayhigh-pressure storage chamber 342 may be constructed such that itsvolume does not change due to, e.g., the high forces and/or torquesresulting from gas that is compressed within the chamber. In an exampleembodiment, the high-pressure storage container 342 may be made of amaterial with a high tensile-strength to weight ratio, such as titaniumor a composite made of spun carbon fiber and epoxy. However,high-pressure storage container 342 may be made of other materials orcombinations of materials, without departing from the scope of theinvention.

In a further aspect, balloon 300 may be configured to generate powerfrom gas flow out of high-pressure storage chamber 342 and into envelope302. For example, a turbine (not shown) may be fitted in the path of thegas flow (e.g., at the end of gas-flow tube 350). The turbine may be agas turbine generator, or may take other forms. Such a turbine maygenerate power when gas flows from high-pressure storage chamber 342 toenvelope 302. The generated power may be immediately used to operate theballoon and/or may be used to recharge the balloon's battery.

In a further aspect, a turbine, such as a gas turbine generator, mayalso be configured to operate “in reverse” in order to pump gas into andpressurize the high-pressure storage chamber 342. In such an embodiment,pump 348 may be unnecessary. However, an embodiment with a turbine couldalso include a pump.

In some embodiments, pump 348 may be a positive displacement pump, whichis operable to pump gas out of the envelope 302 and into high-pressurestorage chamber 342. Further, a positive-displacement pump may beoperable in reverse to function as a generator.

Further, in the illustrated example, the gas-flow system includes avalve 346, which is configured to adjust the gas-flow path betweenenvelope 302, high-pressure storage chamber 342, and fuel cell 344. Inparticular, valve 346 may adjust the gas-flow path such that gas canflow between high-pressure storage chamber 342 and envelope 302, andshut off the path to fuel cell 344. Alternatively, valve 346 may shutoff the path high-pressure storage chamber 342, and create a gas-flowpath such that gas can flow between fuel cell 344 and envelope 302.

Balloon 300 may be configured to operate fuel cell 344 in order toproduce power via the chemical reaction of hydrogen and oxygen toproduce water, and to operate fuel cell 344 in reverse so as to createhydrogen and oxygen from water. Accordingly, to increase its altitude,balloon 300 may run fuel cell 344 in reverse so as to generate gas(e.g., hydrogen gas), which can then be moved into the envelope toincrease buoyancy. Specifically, balloon may increase its altitude byrunning fuel cell 344 in reverse, adjusting valve 346 and valve 352 suchthat hydrogen gas produced by fuel cell 344 can flow from fuel cell 344,through gas-flow tube 350, and into envelope 302.

To run fuel cell 344 “in reverse,” balloon 300 may utilize anelectrolysis mechanism in order to separate water molecules. Forexample, a balloon may be configured to use a photocatalytic watersplitting technique to produce hydrogen and oxygen from water. Othertechniques for electrolysis are also possible.

Further, balloon 300 may be configured to separate the oxygen andhydrogen produced via electrolysis. To do so, the fuel cell 344 and/oranother balloon component may include an anode and cathode that attractthe positively and negatively charged O- and H-ions, and separate thetwo gases. Once the gases are separated, the hydrogen may be directedinto the envelope. Additionally or alternatively, the hydrogen and/oroxygen may be moved into the high-pressure storage chamber.

Further, to decrease its altitude, balloon 300 may use pump 348 to pumpgas from envelope 302 to the fuel cell 344, so that the hydrogen gas canbe consumed in the fuel cell's chemical reaction to produce power (e.g.,the chemical reaction of hydrogen and oxygen to create water). Byconsuming the hydrogen gas the buoyancy of the balloon may be reduced,which in turn may decrease the altitude of the balloon.

It should be understood that variations on the illustrated high-pressurestorage chamber are possible. For example, the high-pressure storagechamber may take on various sizes and/or shapes, and be constructed fromvarious materials, depending upon the implementation. Further, whilehigh-pressure storage chamber 342 is shown as part of payload 306,high-pressure storage chamber could also be located inside of envelope302. Yet further, a balloon could implement multiple high-pressurestorage chambers. Other variations on the illustrated high-pressurestorage chamber 342 are also possible.

It should also be understood that variations on the illustrated air-flowtube 350 are possible. Specifically, any configuration that facilitatesmovement of gas between the high-pressure storage chamber and theenvelope is possible.

Yet further, it should be understood that a balloon and/or componentsthereof may vary from the illustrated balloon 300. For example, some orall of the components of balloon 300 may be omitted. Components ofballoon 300 could also be combined. Further, a balloon may includeadditional components in addition or in the alternative to theillustrated components of balloon 300. Other variations are alsopossible.

3. Balloon Network with Optical and RF Links Between Balloons

In some embodiments, a high-altitude-balloon network may includesuper-node balloons, which communicate with one another via opticallinks, as well as sub-node balloons, which communicate with super-nodeballoons via RF links. Generally, the optical links between super-nodeballoons may be configured to have more bandwidth than the RF linksbetween super-node and sub-node balloons. As such, the super-nodeballoons may function as the backbone of the balloon network, while thesub-nodes may provide sub-networks providing access to the balloonnetwork and/or connecting the balloon network to other networks.

FIG. 4 is a simplified block diagram illustrating a balloon network thatincludes super-nodes and sub-nodes, according to an example embodiment.More specifically, FIG. 4 illustrates a portion of a balloon network 400that includes super-node balloons 410A to 410C (which may also bereferred to as “super-nodes”) and sub-node balloons 420 (which may alsobe referred to as “sub-nodes”).

Each super-node balloon 410A to 410C may include a free-space opticalcommunication system that is operable for packet-data communication withother super-node balloons. As such, super-nodes may communicate with oneanother over optical links. For example, in the illustrated embodiment,super-node 410A and super-node 401B may communicate with one anotherover optical link 402, and super-node 410A and super-node 401C maycommunicate with one another over optical link 404.

Each of the sub-node balloons 420 may include a radio-frequency (RF)communication system that is operable for packet-data communication overone or more RF air interfaces. Accordingly, each super-node balloon 410Ato 410C may include an RF communication system that is operable to routepacket data to one or more nearby sub-node balloons 420. When a sub-node420 receives packet data from a super-node 410, the sub-node 420 may useits RF communication system to route the packet data to a ground-basedstation 430 via an RF air interface.

As noted above, the super-nodes 410A to 410C may be configured for bothlonger-range optical communication with other super-nodes andshorter-range RF communications with nearby sub-nodes 420. For example,super-nodes 410A to 410C may use using high-power or ultra-bright LEDsto transmit optical signals over optical links 402, 404, which mayextend for as much as 100 miles, or possibly more. Configured as such,the super-nodes 410A to 410C may be capable of optical communications atspeeds of 10 to 50 GB/sec or more.

A larger number of balloons may be configured as sub-nodes, which maycommunicate with ground-based Internet nodes at speeds on the order ofapproximately 10 MB/sec. Configured as such, the sub-nodes 420 may beconfigured to connect the super-nodes 410 to other networks and/or toclient devices.

Note that the data speeds and link distances described in the aboveexample and elsewhere herein are provided for illustrative purposes andshould not be considered limiting; other data speeds and link distancesare possible.

In some embodiments, the super-nodes 410A to 410C may function as a corenetwork, while the sub-nodes 420 function as one or more access networksto the core network. In such an embodiment, some or all of the sub-nodes420 may also function as gateways to the balloon network 400.Additionally or alternatively, some or all of ground-based stations 430may function as gateways to the balloon network 400.

4. Illustrative Methods

FIG. 5 is a flow chart illustrating a computer-implemented method,according to an example embodiment. Example methods, such as method 500of FIG. 5, may be carried out by a control system and/or by othercomponents of balloon. A control system may take the form of programinstructions stored on a non-transitory computer readable medium (e.g.,memory 314 of FIG. 3) and a processor that executes the instructions(e.g., processor 312). However, a control system may take other formsincluding software, hardware, and/or firmware.

Example methods may be implemented as part of a balloon's altitudecontrol process. Yet further, example methods may be implemented as partof or in conjunction with a balloon's power generation and/or powermanagement processes.

4a) Altitude Control with Power Management Via Gas Flow

As shown by block 502, method 500 involves a control system causing aballoon to operate in a first mode. While the balloon is operating inthe first mode, the control system may operate the balloon's solar powersystem to generate power for the balloon. As the control system maycause the balloon to use at least some of the power generated by thesolar power system to move gas from the envelope to the high-pressurestorage chamber such that the buoyancy of the balloon decreases, asshown by block 504. At a later point in time, the control system maycause the balloon to operate in a second mode, as shown by block 506.While the balloon is operating in the second mode, the control systemmay move gas from the high-pressure storage chamber to the envelope suchthat the buoyancy of the balloon increases, as shown by block 508.

In an example embodiment, the first mode may be a daytime mode and thesecond mode may be a nighttime mode. Further, the balloon may include abattery in addition to the solar power system. As such, the controlsystem may cause the balloon to operate in the first mode and use thesolar power system during the daytime (e.g., when there is enough sunlight for the solar power system to support some or all of the balloon'sfunctionality), and cause the balloon to operate in the second mode anduse the battery during the nighttime (e.g., when solar power may beunavailable).

In a further aspect, an example method may further involve the controlsystem detecting a predetermined day-night transition condition, andresponsively causing the balloon to transition from the daytime mode tothe nighttime mode. Various day-night transition conditions arepossible. For example, the control system may cause the transition at acertain time (e.g., sunset), or upon detecting that the amount ofsunlight being received by the solar power system has fallen below athreshold level, among other possibilities. Other types of day-nighttransition conditions are also possible.

In some embodiments, the transition between modes may be an immediateswitch from the daytime to nighttime mode. In other embodiments, thetransition between the daytime and nighttime mode may occur over aperiod of time. For example, a balloon's control system may graduallyincrease the amount of battery power being utilized as the amount ofsunlight being received, and thus the amount of power that is beinggenerated by the solar power system decreases. Other examples are alsopossible.

As noted above, an example method may be implemented as part of analtitude control process. As such, altitudinal movements of the balloonmay be achieved in different ways in different modes of operation. Forexample, in the first mode (e.g., daytime mode) the balloon may decreaseits altitude by moving the gas from the envelope to the high-pressurestorage chamber. More specifically, moving gas from the envelope to thehigh-pressure storage chamber reduces the volume in which the gas iscontained and thus increases the density of the gas. Increasing thedensity of the gas reduces the buoyancy of the balloon and thus maycause the balloon to move to a lower altitude. Conversely, when gas ismoved back into the envelope, the density of the gas decreases, which inturn may increase the buoyancy of the balloon and cause the balloon tomove to a higher altitude.

In a further aspect, operation in the second (e.g., nighttime) mode mayinvolve using the gas flow from the high-pressure storage chamber togenerate power. Further, the power that is generated from the gas flowmay be used to power the balloon, or may be used to charge the balloon'sbattery.

During the daytime, a balloon may utilize method 500 to take advantageof available solar power and store the gas that is generated when itdecreases its altitude in the HP pressure, for use to increase itsaltitude during the night. (Note that the balloon may use othertechniques to increase its altitude during the day and/or to decreaseits altitude during the nighttime.) Advantageously, this altitudecontrol process uses a renewable energy source (e.g., the sun) duringthe day, and may also generate power during the night (via gas flow intothe envelope), when solar energy is unavailable.

4b) Altitude Control with Power Management Via Gas Flow and a Fuel Cell

In a further aspect, a balloon may use a fuel cell as part of analtitude control process and/or as part of or in conjunction with aballoon's power management process. In an example embodiment, altitudecontrol and/or power management processes may utilize a fuel cell (orpossibly multiple fuel cells) in combination with the functionality ofhigh-pressure storage chamber described in reference to FIG. 5. Byutilizing both the fuel cell and movement of gas between the envelopeand high-pressure storage chamber, a balloon may be provided with twoways of increasing buoyancy including one that generates power and onethat uses power, and two ways of decreasing buoyancy including one thatgenerates power and one that uses power.

For example, FIG. 6 is a combined operational state diagram 600illustrating the functions of a balloon that utilizes a fuel cell and agas-flow system for altitude control, according to an exampleembodiment. Operating in a manner such as that illustrated by FIG. 6 mayfacilitate power management via use of both the fuel cell and gas flowbetween the envelope and the high-pressure storage chamber for buoyancyadjustments in a manner that helps the balloon to generate and utilizepower in a more efficient manner.

More specifically, a balloon may operate in a daytime mode 602 and anighttime mode 604. A balloon may transition between the daytime mode602 and the nighttime mode 604, as shown by transitions 603 and 605.

Further, the balloon may transition between the daytime mode 602 and thenighttime mode 604 when it detects certain conditions (e.g., a certaintime of day, the availability of a certain amount of sunlight (or lackthereof), etc.). Alternatively, a balloon may be instructed when totransition between the daytime mode 602 and the nighttime mode 604 byanother entity, such as another balloon in the balloon network or aground-based station, for instance.

A balloon may implement various processes that involve altitudinaladjustment, such as various types of station-keeping processes, wherealtitudinal movements take advantage of altitudinally varying winds toachieve longitudinal and latitudinal movement, for instance. Accordingto an example embodiment, the daytime mode 602 and the nighttime mode604 the may involve different techniques for increasing the balloon'saltitude, and different techniques for decreasing the balloon'saltitude.

In particular, while operating in the daytime mode 602, the balloon maydetermine that the balloon should change its altitude, as shown by block606 (shown within daytime mode 602). If the balloon determines it shouldmove to a lower altitude, the balloon may decrease its buoyancy bymoving gas out of the envelope and into the high-pressure storagechamber, as shown by block 608. To do so, the balloon may use power thatis generated by its solar power system. In particular, the balloon mayuse power supplied by its solar power system to operate a pump, and pumpgas from the envelope into the high-pressure storage chamber.

Further, if the balloon determines it should move to a higher altitudewhile the balloon is operating in the daytime mode 602, then the balloonmay increase its buoyancy by using its fuel cell to produce gas (e.g.,hydrogen gas), and directing the gas that is produced into the envelope,as shown by block 610. For example, the balloon may operate its fuelcell in reverse to produce hydrogen gas and oxygen gas from water. Thehydrogen gas may then be moved (or allowed to move) into the balloon'senvelope so as to increase buoyancy by an amount that corresponds to adesired increase in altitude. Further, in daytime mode 602, the balloonmay utilize power from its solar power system to operate its fuel cellin reverse.

Further, while operating in the nighttime mode 604, the balloon may alsodetermine that the balloon should change its altitude, as shown by block606 (shown again within nighttime mode 604). However, altitudeadjustments may be accomplished differently in nighttime mode 604. Inexample embodiment, if the balloon determines it should move to a loweraltitude, the balloon may decrease its buoyancy by operating its fuelcell so as to use the gas from the envelope to generate power for theballoon, as shown by block 612. For example, if the balloon useshydrogen gas as a lifting gas, the fuel cell may cause a chemicalreaction between oxygen gas and hydrogen gas from the envelope, whichproduces water. The chemical reaction also generates power, which may beused to power the balloon while the balloon is operating in nighttimemode 604 and/or may be used to recharge the balloon's battery.

If the balloon determines it should move to a higher altitude while theballoon is operating in the nighttime mode 604, then the balloon mayincrease its buoyancy by moving gas out of the high-pressure storagechamber and into the envelope. To do so, the balloon may, for example,open a valve and allow gas that is stored in the high-pressure storagechamber to rise into the envelope. Other techniques for moving gas fromthe high-pressure storage chamber to the envelope are also possible.

Note that a balloon may determine that it should change altitude atvarious points in time, while operating in the daytime mode and/or whileoperating in the night-time mode. Accordingly, block 608 and/or block610 may each be carried out a number of times while the balloon isoperating in daytime mode 602. Similarly, block 612 and/or block 614 mayeach be carried out a number of times while the balloon is operating indaytime mode 602 However, it is also possible that block 608 and/orblock 610 might not be carried out at all during a single period inwhich the balloon operates in daytime mode 602 (e.g., during a singleday).

In a further aspect, note that gas could be moved between the envelopeand the high-pressure storage chamber for reasons other than adjustingbuoyancy and altitude control. For example, gas could be moved betweenthe envelope and the high-pressure storage chamber as part of a pressuremanagement process. In particular, as the envelope heats up and coolsdown, the pressure inside it will be much higher when the air is hot(e.g., mid-day) than when it is cold (e.g. the middle of the night).This could create a scenario where heat causes the air pressure insidethe envelope to increase so much as to present a risk of popping theenvelope. Accordingly, to regulate the pressure, a balloon may move somegas from the envelope to the high-pressure storage chamber. Further, theballoon could move gas back into the envelope at when it is morestructurally safe to do so (e.g., after the sun sets and the temperatureof the gas in the envelope decreases). Other examples are also possible.

5. Conclusion

The above detailed description describes various features and functionsof the disclosed systems, devices, and methods with reference to theaccompanying figures. While various aspects and embodiments have beendisclosed herein, other aspects and embodiments will be apparent tothose skilled in the art. The various aspects and embodiments disclosedherein are for purposes of illustration and are not intended to belimiting, with the true scope and spirit being indicated by thefollowing claims.

What is claimed is:
 1. A computer-implemented method comprising: causing a balloon to operate in a first altitude-control mode, wherein the balloon comprises an envelope, a high-pressure storage chamber, and a solar power system; while the balloon is operating in the first altitude-control mode: operating the solar power system to generate power for the balloon; determining that the balloon should move in a first horizontal direction; determining that wind at a lower altitude corresponds to the first horizontal direction; in response to determining that wind at the lower altitude corresponds to the first horizontal direction, determining that the balloon should move to the lower altitude; and responsively using at least some of the power generated by the solar power system to move gas from the envelope to the high-pressure storage chamber such that the buoyancy of the balloon decreases; causing the balloon to operate in a second altitude-control mode; and while the balloon is operating in the second altitude-control mode, determining that the balloon should move to a higher altitude and responsively moving gas from the high-pressure storage chamber to the envelope such that the buoyancy of the balloon increases.
 2. The method of claim 1: wherein operation in the first altitude-control mode further comprises moving the gas from the envelope to the high-pressure storage chamber in order to decrease altitude of the balloon; and wherein operation in the second altitude-control mode further comprises moving gas from the high-pressure storage chamber to the envelope in order to increase the altitude of the balloon.
 3. The method of claim 1, wherein the balloon further comprises a battery, and wherein operation in the second altitude-control mode further comprises using power supplied by the battery.
 4. The method of claim 1, wherein determining that the balloon should move to the higher altitude comprises: determining that the balloon should move in a second horizontal direction; determining that wind at the higher altitude corresponds to the second horizontal direction; and in response to determining that wind at the higher altitude corresponds to the second horizontal direction, determining that the balloon should move to the higher altitude.
 5. The method of claim 1, wherein the first altitude-control mode is a daytime mode and the second altitude-control mode is a nighttime mode.
 6. The method of claim 5, further comprising: detecting a predetermined day-night transition condition; and responsively causing the balloon to transition from operation in the daytime mode to operation in the nighttime mode.
 7. The method of claim 1, wherein using at least some of the power generated by the solar power system to move gas from the envelope to the high-pressure storage chamber such that the buoyancy of the balloon decreases is performed during a first time period, the method further comprising: while the balloon is operating in the first mode and during a second time period, determining that the balloon should move to a higher altitude and responsively: operating a fuel cell of the balloon in reverse to produce gas; and moving the gas produced by the fuel cell to the envelope such that the buoyancy of the balloon increases; and while the balloon is operating in the second mode, determining that the balloon should move to a lower altitude and responsively: moving gas from the envelope to the fuel cell; and operating the fuel cell so as to use the gas from the envelope to generate power for the balloon, such that the buoyancy of the balloon decreases.
 8. The method of claim 7, further comprising: moving the gas produced by the fuel cell to the envelope in order to increase the altitude of the balloon; and operating the fuel cell so as to use the gas from the envelope to generate power in order to decrease the altitude of the balloon.
 9. A non-transitory computer readable medium having stored therein instructions executable by a computing device to cause the computing device to perform functions comprising: causing a balloon to operate in a first altitude-control mode, wherein the balloon comprises an envelope, a high-pressure storage chamber, and a solar power system; while the balloon is operating in the first altitude-control mode: operating the solar power system to generate power for the balloon; determining that the balloon should move in a first horizontal direction; determining that wind at a lower altitude corresponds to the first horizontal direction; in response to determining that wind at the lower altitude corresponds to the first horizontal direction, determining that the balloon should move to the lower altitude; and responsively using at least some of the power generated by the solar power system to move gas from the envelope to the high-pressure storage chamber such that the buoyancy of the balloon decreases; causing the balloon to operate in a second altitude-control mode; and while the balloon is operating in the second altitude-control mode, determining that the balloon should move to a higher altitude and responsively moving gas from the high-pressure storage chamber to the envelope such that the buoyancy of the balloon increases.
 10. The non-transitory computer readable medium of claim 9, wherein the first altitude-control mode is a daytime mode and the second altitude-control mode is a nighttime mode.
 11. The non-transitory computer readable medium of claim 9: wherein operation in the first altitude-control mode further comprises moving the gas from the envelope to the high-pressure storage chamber in order to decrease altitude of the balloon; and wherein operation in the second altitude-control mode further comprises moving gas from the high-pressure storage chamber to the envelope in order to increase the altitude of the balloon.
 12. The non-transitory computer readable medium of claim 9, wherein using at least some of the power generated by the solar power system to move gas from the envelope to the high-pressure storage chamber such that the buoyancy of the balloon decreases is performed during a first time period, and wherein the functions further comprise: while the balloon is operating in the first mode and during a second time period, determining that the balloon should move to a higher altitude and responsively: operating a fuel cell of the balloon in reverse to produce gas; and moving the gas produced by the fuel cell to the envelope in order to increase the buoyancy of the balloon; and while the balloon is operating in the second mode, determining that the balloon should move to a lower altitude and responsively: moving gas from the envelope to the fuel cell; and operating the fuel cell so as to use the gas from the envelope in order to generate power for the balloon and decrease the buoyancy of the balloon.
 13. The non-transitory computer readable medium of claim 12, wherein the functions further comprise: moving the gas produced by the fuel cell to the envelope in order to increase the altitude of the balloon; and operating the fuel cell so as to use the gas from the envelope to generate power in order to decrease the altitude of the balloon.
 14. A balloon comprising: a solar power system configured to generate power for a balloon, wherein the balloon comprises an envelope and a high-pressure storage chamber; and a control system that is configured to cause the balloon to operate in at least a first altitude-control mode and a second altitude-control mode; wherein, during operation in the first altitude-control mode, the control system is configured to: operate the solar power system to generate power for the balloon; determine that the balloon should move in a first horizontal direction; determine that wind at a lower altitude corresponds to the first horizontal direction in response to the determination that wind at the lower altitude corresponds to the first horizontal direction, determine that the balloon should move to the lower altitude; and responsively use at least some of the power generated by the solar power system to move gas from the envelope to the high-pressure storage chamber such that the buoyancy of the balloon decreases; wherein, during operation in the first altitude-control mode, the control system is configured to determine that the balloon should move to a higher altitude and responsively move gas from the high-pressure storage chamber to the envelope such that the buoyancy of the balloon increases.
 15. The balloon of claim 14, further comprising a battery, wherein the control system is further configured to balance use of power from the battery with use of power generated by the solar power system.
 16. The balloon of claim 14, further comprising: a fuel cell configured to generate power for the balloon; wherein the control system is further configured to: while the balloon is operating in the first mode, determine that the balloon should move to a higher altitude and responsively: operate the fuel cell of the balloon in reverse to produce gas; and move the gas produced by the fuel cell to the envelope such that the buoyancy of the balloon increases; and while the balloon is operating in the second mode, determine that the balloon should move to a lower altitude and responsively: move gas from the envelope to the fuel cell; and operate the fuel cell so as to use the gas from the envelope to generate power for the balloon, such that the buoyancy of the balloon decreases.
 17. The balloon of claim 14, wherein the first altitude-control mode is a daytime mode and the second altitude-control mode is a nighttime mode.
 18. The balloon of claim 14, further comprising a turbine system that is operable to generate power via gas flow out of the high-pressure storage chamber through the turbine and into the envelope.
 19. A computer-implemented method comprising: causing a balloon to operate in a first altitude-control mode, wherein the balloon comprises an envelope, a high-pressure storage chamber, and a solar power system; while the balloon is operating in the first altitude-control mode: operating the solar power system to generate power for the balloon; determining that the balloon should move in a first horizontal direction; determining that wind at a lower altitude corresponds to the first horizontal direction; in response to determining that wind at the lower altitude corresponds to the first horizontal direction, determining that the balloon should move to the lower altitude; and responsively using at least some of the power generated by the solar power system to move gas from the envelope to the high-pressure storage chamber; causing the balloon to operate in a second altitude-control mode; and while the balloon is operating in the second altitude-control mode, determining that the balloon should move to a higher altitude and responsively moving gas from the high-pressure storage chamber to the envelope.
 20. The method of claim 19, wherein the balloon further comprises a battery, and wherein operation in the second altitude-control mode further comprises using power supplied by the battery.
 21. The method of claim 19, wherein using at least some of the power generated by the solar power system to move gas from the envelope to the high-pressure storage chamber is performed during a first time period, the method further comprising: while the balloon is operating in the first altitude-control mode and during a second time period, determining that the balloon should move to a higher altitude and responsively: operating a fuel cell of the balloon in reverse to produce gas; and moving the gas produced by the fuel cell to the envelope; and while the balloon is operating in the second altitude-control mode, determining that the balloon should move to a lower altitude and responsively: moving gas from the envelope to the fuel cell; and operating the fuel cell so as to use the gas from the envelope to generate power for the balloon.
 22. The method of claim 19: wherein gas is moved from the envelope to the high-pressure storage chamber in order to reduce the pressure exerted on the envelope by gas that is inside the envelope; and wherein gas is moved from the high-pressure storage chamber to the envelope in order to increase the pressure exerted on the envelope by gas that is inside the envelope. 